Nanostructures, such as nano-islands and nanowires, are important for advanced electronic, magnetic and optical devices because of the unique characteristics of nanoscale structures. The term “nanostructure” as used herein refers to a structure having an extent of less than a micron in at least one of its three dimensions. The term “nanowire” refers to a structure having an extent of less than a micron in at least two of its three dimensions. It is used generically to include both solid nanowires and hollow nanowires (nanotubes). The term “nanoislands” refers to substrate-supported structures having an extent of less than a micron in at least two and preferably in all three dimensions. Small diameter nanowires, such as carbon nanotubes with a diameter on the order of 1–100 nanometers, have received considerable attention in recent years. See Liu et al., SCIENCE, Vol. 280, p. 1253 (1998); Ren et al., SCIENCE, Vol. 282, p. 1105 (1998); Li et al., SCIENCE, Vol. 274, p. 1701 (1996); J. Tans et al., NATURE, Vol. 36, p. 474 (1997); Fan et al., SCIENCE, Vol. 283, p. 512 (1999); Bower et als., Applied Physics Letters, Vol. 77, p. 830 (2000), and Applied Physics Letters, Vol. 77, p. 2767 (2000).
Carbon nanotubes exhibit unique atomic arrangements, nano-scale structures, and unusual physical properties such as one-dimensional electrical behavior, quantum conductance, and ballistic transport. Carbon nanotubes are one of the smallest dimensioned nanowire materials with generally high aspect ratio and small diameter, e.g., single-wall nanotubes may be made with diameters of ˜1 nm and multi-wall nanotubes with diameters of less than ˜50 nm.
High-quality single-walled carbon nanotubes are typically grown as randomly oriented, needle-like or spaghetti-like, tangled nanowires by laser ablation or arc techniques. Chemical vapor deposition (CVD) methods such as used by Ren et al., Fan et al., Li et al., and Bower et al. tend to produce multiwall nanowires attached to a substrate, often with aligned, parallel growth perpendicular to the substrate. As described in these articles, catalytic decomposition of hydrocarbon-containing precursors such as ethylene, methane, or benzene produces carbon nanotubes when the reaction parameters such as temperature, time, precursor concentration, flow rate, are optimized. Nucleation layers such as thin coatings of Ni, Co, or Fe, are often intentionally added to the substrate surface to nucleate a multiplicity of isolated nanowires. Carbon nanotubes can also be nucleated and grown on a substrate without using a metal nucleating layer, e.g., by using a hydrocarbon-containing precursor mixed with a chemical component, such as ferrocene (C5H5)2Fe, which contains one or more catalytic metal atoms. During the chemical vapor decomposition, these metal atoms serve to nucleate nanotubes on substrate surface. See Cheng et al., CHEM. PHYSICS LETTERS, Vol. 289, p. 602 (1998), and Andrews et al., CHEM. PHYSICS LETTERS, Vol. 303, p. 467 (1999).
Carbon nanotubes are useful for field emission devices such as flat panel field emission displays, microwave amplifiers, and electron beam lithography devices. Conventional field emission cathode materials typically have been made of metal (such as Mo) or semiconductor material (such as Si) with sharp tips of submicron size. However, the control voltage required for emission is relatively high (around 100 V) because of high work functions and insufficiently sharp tips. To significantly enhance local fields and reduce the voltage requirement for emission, it would be advantageous to provide nanoscale cathodes with small diameters and sharp tips.
In field emission devices, unaligned, randomly distributed nanowires are inefficient electron emitters due to the varying distance and hence varying local electric fields between the cathode (emitting nanowire tips) and the gate or anode. In addition, when unaligned nanowires are used for emitters, an applied electric field between anode and cathode bends the nanowires. The degree of bending is dependent on the applied voltage. This bending causes uncontrollable and undesirable changes in the distance between cathode and gate, and hence alters the local field on different nanowires. In some cases, the bending causes outright electrical shorting between the nanowire tips and the gate. Nanowires pre-aligned toward the anode could prevent or reduce the bending problem.
Referring to the drawings, FIGS. 1(a) and 1(b) schematically illustrate conventional configurations of aligned nanotubes 10 grown on a substrate 11 in a dense “forest-like” configuration (FIG. 1(a)) or in spaced-apart “forests” (FIG. 1(b)). The present invention is directed to more desirable configurations of more widely spaced-apart individual nanostructures (FIG. 1c) or spaced apart small groups of nanostructures (FIG. 1(d)). A forest configuration wastes the unique, high-aspect-ratio, field concentrating characteristics of individual nanowires. While the alignment of nanowires is important for many applications, highly oriented nanowires do not alone guarantee efficient field emission. The reason is that the nanowires are so closely spaced that they shield each other from effective field concentration at the ends. It is therefore desirable to provide more widely spaced apart configurations of nanostructures as are schematically illustrated in FIGS. 1(c) and 1(d).